In the background description to follow, the present invention is described in relation to magnetic resonance (MR) imaging techniques used in the examination and diagnosis of the prostate gland, although the invention also may have application to MR imaging of tissues in other body cavities.
Carcinoma of the prostate is the second most common malignancy in the male population, and it also has the second highest death rate. Diagnostic blood tests for prostatic carcinoma are imperfect and the disease is often well advanced by the time it is first diagnosed. The traditional methods of diagnosing carcinoma of the prostate are mainly digital rectal examination and estimation of serum acid phosphatase levels. The digital examination remains the most useful diagnostic technique inasmuch as acid phosphatase activity can be normal while the tumor is still confined to the gland.
Sonographic examination has been reported to have good diagnostic results in the detection of carcinoma. However, benign diseases such as benign prostatic hypertrophy and chronic prostatitis can yield similar sonographic appearances.
Magnetic resonance imaging has recently been shown to produce high quality images of the human body. Briefly, MR imaging makes use of magnetic fields and radio frequency waves to generate intensity-modulated images from specific sections of the body. MR imaging systems generally include a large magnet for generating a magnetic field. The patient being analyzed is exposed to the magnetic field of the magnet. Hydrogen nuclei (protons) in the magnetic field resonate when exposed to radio waves of a correct frequency. For imaging purposes, the strong uniform magnetic field of the magnet is selectively altered in one or more directions, preferably by small magnetic fields produced by three separate gradient coils associated with the magnet. Current passing through the gradient coils linearly alters the magnetic field of the magnet in directions controlled by the gradient coils. Signal transmission and reception are produced through use of a radio frequency (RF) transmitter coupled to a transmitting coil or antenna within the imaging unit and an RF receiver coupled to a receiving coil or antenna also located in the imaging unit. The receiving coil is positioned as close to the patient as possible for maximum imaging sensitivity. The patient is often surrounded by a body coil which serves both as a transmitting and receiving antenna. Alternatively, the body coil can be used as a transmitting antenna only, and a separate surface coil is used as a receiving antenna. The surface coil can usually be placed closer to the tissues under examination than a single body coil. An RF oscillator generates radio waves of different frequencies. By controlling the magnetic field in a known way through a switching system that controls the current in the gradient coils, and by generating radio waves of a select frequency, the exact location at which the patient's body is imaged can be controlled. When the frequency of the RF signal is set for the exact value of the magnetic field, resonance occurs. Radio waves of the same frequency are emitted from the portion of the patient being imaged, which induces small currents in the receiving coil. The induced currents are detected to produce an output signal dependent upon the number of protons involved in the resonance and tissue-specific parameters T-1 and T-2. The variation in proton density in different areas of the patient's body produces good contrast in an MR image and is therefore useful in differentiating among different tissues of the human body. The output signal from the RF receiver is processed by a computer system to produce an image display so that clinical diagnosis can be made by visual inspection of the displayed image. The quality of the image display is critical. It is desirable to obtain an image having high resolution so that clinical diagnosis can be as precise as possible. High resolution imaging is also critical in detecting tumor growth at its earlier stages where treatment of the disease is still possible.
Use of a body coil as both the transmitter and receiver antenna yields MR images of the body which can be useful in many clinical situations. However, use of anatomically shaped surface coils for signal detection yields images with higher signal-to-noise ratio in comparison to the usual body coil. As a result, the surface coils yield MR images with much higher sensitivity and therefore more detail in the critical anatomical areas. Such surface coils are used for RF detection only and excitation is produced by the standard body coil. These surface coils have been successful in the past in obtaining reasonably good MR images for exterior anatomical regions of the patient's body.
Use of a body coil (as both the transmitter and receiver coil) has not yielded high resolution images of the prostate gland. Higher resolution imaging of the prostate is desirable to obtain high detail images for detecting prostatic carcinoma or benign prostatic hypertrophy at their earlier stages. The prostate gland tissue has a capsule which forms an outer wall of the gland. Present MR imaging techniques detect prostatic carcinoma, but only after it is too far advanced, where the cancer has passed through the capsule and invaded surrounding tissues. To date, MR imaging techniques have not developed high resolution images that have been shown to detect early malignant tumors within the prostate gland routinely before they have passed through the outer capsule and invaded surrounding structures.
The present invention provides an improved MR imaging system that produces high resolution images of the prostate and tissues within other body cavities. With the present invention, detection of prostatic carcinoma and other malignant tumors at their earlier stages of growth is possible, when compared with use of a single body coil as both the transmitter and receiver in the MR imaging system.